1. Field of the Invention
The invention relates to a process for directly bonding a metal component, the surface of which is covered by an oxide layer, to a ceramic substrate, by heating the ceramic substrate with the metal component placed thereon, up to a temperature above the eutectic point of the metal and metal oxide but below the melting point of the metal. The process can be used, for example, for bonding copper foils, in particular large-area foils.
2. Description of the Prior Art
Process for the direct bonding of metal to ceramics have been disclosed in German published specifications No. 2,633,869, No. 2,319,854 and No. 2,508,244 (corresponding to U.S. Pat. Nos. 3,994,430, 3,766,634 and 3,911,553, respectively).
In German published specification No. 2,633,869, a process has been described in which an oxidized copper foil and an oxide-ceramic substrate are heated in an inert atmosphere at a temperature between the eutectic point of copper and copper oxide and the melting point of copper, until a sub-eutectic melt between the copper and the substrate has formed. The heating in an inert atmosphere, however, does not give the desired strong adhesive bond between the copper and the ceramic, since, at the given process temperatures, Cu.sub.2 O is reduced to Cu, which does not bond to ceramics, unless a defined minimum oxygen partial pressure is maintained. In the published specification quoted, it is also proposed to use an oxygen-containing copper material without an oxide coating, in place of preoxidized copper. Apart from the fact that even here a bond between the copper and the ceramics cannot be formed, unless a minimum oxygen partial pressure is maintained in the furnace atmosphere, tests have shown that the oxygen dissolved in the metal diffuses to the contact face between the copper and the ceramic and remains occluded there; this leads to the formation of blisters and hence to a defective bond between the metal and the ceramic.
The process described in German published specification No. 2,319,854 differs from that just described in that a bright metal foil, in particular of copper, which has not been preoxidized, is to be bonded to the oxide-ceramic substrate in a reactive atmosphere, in particular an atmosphere containing oxygen. In this process, the oxygen in the atmosphere must first oxidize the copper superficially, before the actual bonding process can proceed. It has been found, however, that, in particular in the case of large-area bonds between ceramics and copper, the oxygen cannot penetrate in an adequate quantity into the gap between the ceramics and copper, so that blister-like points without adhesion result. Moreover, there is a risk of the copper foil, at the indicated quantities of oxygen, being coated with a thick black copper oxide layer on the side facing away from the ceramic substrate, and this coating has to be removed in an additional working step, before further processing is possible.
In the process described in German published specification No. 2,508,244, a preoxidized copper foil is initially bent in such a way that it is placed only gradually onto the ceramic substrate during heating. In this way, any gas bubbles are intended to be forced out outwards. This process is expensive and involved, and it is not always applicable if the shapes of the metal components or of the ceramics are complicated, or the metal component does not place itself uniformly onto the substrate. Moreover, event with the quantity of oxygen indicated in the published specification, the copper surface not resting on the substrate is covered with a thick black oxide layer which afterwards must be removed.